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UBC Theses and Dissertations

Surface science studies of organosilane and zinc phosphate coatings on 2024-T3 aluminum alloy Susac, Darija

Abstract

The effects of 2024-T3 alloy microstructure on the formation of organosilane and zinc phosphate (ZPO) coatings have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) , energy dispersive X-ray (EDX ) spectroscopy and scanning Auger microscopy (SAM). Some studies of corrosion protection were also made using electrochemical polarization measurements, especially for comparing different post-treatment rinses applied to the ZPO coatings. This thesis describes new information for the different chemistries occurring at microregions of 2024-A1 alloy, especially those associated with intermetallic compounds (secondphase particles) of compositions Al-Cu-Mg and Al-Cu-Fe-Mn. These particles have been shown to strongly affect initiation of the ZPO coatings. Coating crystallites start to form at Al-Cu-Mg particles, while the coverage on the Al-Cu-Fe-Mn particles is more amorphous. During the etching stage o f the coating process, the natures of the Al-Cu-Mg particles appear to change from anodic to cathodic character, and this helps drive the precipitation of ZPO crystals. Additionally, a Cu enrichment is found at the interface between the Al-Cu-Mg particles and the ZPO coating. After completion, the coating thickness is least on the matrix regions closest to the Al-Cu-Mg particles, where the competition between substrate etching and ZPO deposition is strongest. A lowering in the pH of the coating solution results in thinner and more amorphous ZPO coatings, and a greater Cu enrichment. The adsorption of bis-l,2-(triethoxysilyl)ethane (BTSE) silane is also strongly affected by the distribution o f second-phase particles and by the amount of oxide at the 2024-A1 alloy surface. For a mechanically-polished sample, BTSE adsorbs on the matrix regions away from the particles, and on the particles themselves, but less adsorbs near the particle-matrix interface areas. For an air-oxidized sample, the amount o f adsorbed BTSE increases markedly, but with a high degree of non-uniformity. It is postulated that the BTSE deposition depends on the precise natures of oxide layer formed at the various micro-areas, and on the competition between silane deposition and oxide etching. In contrast, y-aminopropyltriethoxysilane (y-APS) forms a film of relatively even thickness over a large sample area, apparently due to the formation of hydrogen bonds through the amino groups.

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